Arm backend: Improve pre-push hook (#9873)

Now checks:
* License header
* Lintrunner
* Commit message

This is done for all commits that are not on the remote branch the
current branch is tracking.

Commit message checks are:
* Subject limit is 72 characters
* Subject should start with "Arm backend: " + an imperative verb
* Body limit is 72 characters
* Must contain Signed-off-by

Lines can be shorter than 72, but unless it's a new paragraph each line
should get to as close to 72 characters as possible. This is not checked
by the hook.

Signed-off-by: Sebastian Larsson <sebastian.larsson@arm.com>
Signed-off-by: Yufeng Shi <yufeng.shi@arm.com>